1. FIELD OF THE INVENTION
The invention relates to a process and apparatus for the regeneration of fluidized catalytic cracking catalyst.
2. DESCRIPTION OF RELATED ART
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts. A good overview of the importance of the FCC process, and its continuous advancement, is reported in Fluid Catalytic Cracking Report, Amos A. Avidan, Michael Edwards and Hartley Owen, as reported in the Jan. 8, 1990 edition of the Oil & Gas Journal.
Modern catalytic cracking units use active zeolite catalyst to crack the heavy hydrocarbon feed to lighter, more valuable products. Instead of dense bed cracking, with a hydrocarbon residence time of 20-60 seconds, much less contact time is needed. The desired conversion of feed can now be achieved in much less time, and more selectively, in a dilute phase, riser reactor.
There has been considerable evolution in the design of FCC units, which evolution is reported to a limited extent in the Jan. 8, 1990 Oil & Gas Journal article. Many FCC regenerator designs are used, most of which involve bubbling dense bed regenerators. There are two generic types of regenerators: high efficiency units, operating with a fast fluidized bed and bubbling dense bed units. Three species have evolved of bubbling dense bed units:
1. Cross-flow
2. Swirl
3. Orthoflow.
The cross-flow and swirl regenerators have severe NOx problems and capacity. The NOx and capacity problems are an inherent by-product of bubbling fluidized bed operation. Large amounts of regeneration gas bypass the fluidized bed in the form of large bubbles. There are localized high oxygen concentrations, and any nitrogen containing coke burned there forms NOx. Much CO is produced from oxygen starved regions of the bed, and this CO mixes with the oxygen rich bubbles to cause afterburning in dilute phase regions of the bed. Additional amounts of NOx can form in the dilute phase, especially when afterburning is severe. In addition, the beds are made so large, due to inefficient contacting of gas and solids, that some portions of the bed are stagnant so much of the bed remains for too long in the regenerator and discharges oxygen rich flue gas into the dilute phase region.
The cross-flow regenerators have similar problems, but usually form somewhat less NOx than swirl regenerators.
Both swirl and cross-flow regenerators do a good job at retaining catalyst and fines. This is fairly easy to do in such regenerators, because the relatively low superficial vapor velocities (which promote formation of undesired large bubbles in the dense bed) do not entrain catalyst as much as the higher superficial vapor velocities used in high efficiency regenerators.
The problems of poor flow in the dense bed of swirl and cross-flow regenerators can be largely solved by putting in baffles, or multiple inlets or outlets, as taught in U.S. Pat. Nos. 4,980,048 (cross-flow), 4,994,424 (swirl) incorporated by reference.
These units still produce more NOx than desired, and attempts at increasing capacity, by blowing more air in them, increase the dilute phase traffic sufficiently to cause a dust emissions problems in some units.
The Kellogg Ultra Orthoflow converter, Model F, shown in FIG. 1 of this patent application, and also shown as FIG. 17 of the Jan. 8, 1990 Oil & Gas Journal article discussed above, has a large, bubbling dense bed regenerator with few stagnant regions, as catalyst is added and dispersed through a centrally located catalyst standpipe.
These units, like the other bubbling dense bed regenerators discussed above, cannot easily tolerate more combustion air without increasing dilute phase catalyst traffic. We recently suggested a way to achieve the benefits of FFB coke combustion, while retaining most of the original design. We were able, in predecessor applications (one of which is now U.S. Pat. No. 5,047,140, incorporated by reference) to obtain some improvements in this design with a side mounted fast fluidized bed coke combustor, which discharged catalyst into an annular region about the stripper catalyst standpipe. The '140 patent permitted an increase in regenerator duty, without a proportionate increase in superficial vapor velocity in the bubbling dense bed, but our approach required considerably mechanical modification of the unit to provide an annular return region about the stripper catalyst standpipe and relied on an increase in catalyst traffic near the stripper standpipe to increase the heat transfer coefficient. Although preheating of spent catalyst in the stripper standpipe is beneficial, the design would increase dilute phase catalyst traffic. The separator design shown recovered only "a majority of the catalyst . . ." and would thus increase particulate loading in the dilute phase. We wanted a lower cost way to increase the regeneration capacity of these Orthoflow units.
The bubbling bed regenerators discussed above all generally size the cyclones and the regenerator vessel to deal with catalyst entrainment expected from a given superficial vapor velocity through the bubbling dense bed. Not all regenerators are limited to dense bed regeneration, and these fast fluidized bed regenerators will be reviewed next.
High efficiency regenerators inherently make less NOx than bubbling bed regenerators, but even these may make more NOx than desired. These regenerators also present special problems, in that increases in superficial vapor velocity in the regenerator may remove more inventory than desired from the fast fluidized bed coke combustor. Thus, attempts to increase the capacity of these units by increasing air flow to the coke combustor can reduce residence time of spent catalyst in the coke combustor. This can be offset by increased catalyst recycle to the coke combustor from the second dense bed, but increased catalyst recycle leads to increased catalyst traffic and increased dust emissions, and some of these units may have difficulty complying with strict local limits on particulates.
Local limits on particulates emissions can be so severe, and operation so sensitive to regenerator superficial vapor velocity, that particulates emissions can limit throughput.
One refiner, with a high efficiency regenerator, recently reported on use of oxygen enrichment to reduce vapor velocity. Liquid oxygen was vaporized by passing through a cooling tower, then mixed with air from the air blower. "See O2 enrichment increases FCC operating flexibility," OGJ, May 11, 1992, p. 40. Other refiners have resorted to oxygen addition in the past, usually to overcome air blower limitations, but sometimes to deal with velocity limitations.
To summarize, the current regenerator designs can be arbitrarily classified as relying on either a bubbling dense bed or a fast fluidized bed for regeneration. Some hybrid approaches exist which are reviewed hereafter. This should not be construed as an exhaustive search of hybrid regenerators, but rather as a representative sampling.
U.S. Pat. No. 3,821,103, Owen, discloses in FIG. II a bubbling bed regenerator with a riser regenerator 62 passing up through a bubbling dense bed 78 and terminating in a riser cyclone 70. Catalyst regeneration then continued in the bubbling dense bed. The two flue gas streams were kept isolated. The sulfur rich coke was burned in the riser regenerator, and the sulfur rich flue gas removed via conduit 82, while dense bed flue gas was removed via conduit 98. Such an approach would not add to the dilute phase catalyst loading above dense bed region 78, as no gas or entrained catalyst from the riser regenerator entered the dilute phase region. While the approach is certainly valid, it requires considerable capital expenditure to implement this, and can not be readily retrofit into existing regenerators.
U.S. Pat. No. 3,866,060, Owen, discloses in FIG. II a relatively dilute phase zone 63 operating within a regenerator vessel containing a bubbling fluidized bed of catalyst disposed as an annulus about zone 63. The Figure also shows that catalyst entrainment in the dilute phase region is greatly increased by this approach.
It is believed that most hybrid approaches follow similar paths, i.e., combinations of high superficial vapor velocity regeneration and bubbling bed regeneration either lead to increased catalyst traffic or require considerable capital expense or both.
We recognized the need for a better regenerator design, one which would increase the coke burning capacity of regenerators without proportionately increasing dust emissions, and without requiring undue capital expense.
We discovered a way to increase coke burning capacity of bubbling dense bed regenerators without proportionately increasing dense bed catalyst entrainments. We were even able to reduce NOx emissions, while increasing coke burning.
As applied to high efficiency regenerators, we could increase the amount of coke, especially of nitrogenous coke, burned in an oxidizing atmosphere upstream of the dilute phase transport riser, without appreciable shrinkage of spent catalyst residence time in the coke combustor.